When wells such as oil and gas wells are drilled, a drilling fluid, commonly known as ‘mud’ is circulated through the well to assist in removing drilled material, to stabilise the wall of the well and to balance the pressure of fluids in the formation through the well. At various points during the drilling of the well, and once the well drilling is completed, casings are typically cemented in the well. This is achieved by positioning the casing in the well and pumping a cement slurry down the casing from the surface and back up the annulus between the casing and the wall of the well where it is allowed to set to support the casing and provide zonal isolation.
In cementing operations, drilling mud remaining in the well needs to be cleaned up before pumping cement to improve the bonding of cement to casing and wellbore. The cleanup is typically done by pumping a chemical wash with surfactant solutions through the well. Following this a spacer is pumped through the well to displace the mud ahead of the cement slurry. The spacer needs to have a proper rheology profile to displace the mud and chemical wash fluid and carry particles remaining in the well. Since the spacer comes into contact with both the mud and cement, it needs to have some compatibility with both materials. For the mud side, the spacer fluid should be compatible with the mud particles, brine and solvents. For cement side, the fluid should be able to tolerate high pH, calcium and other cementing additives. By itself, the spacer also needs to be able to be weighted with brines or solid weighting agents to maintain the hydrostatic pressure in the well.
Visco-elastic surfactants (VES) have previously been proposed for well treatment fluids, particularly in the field of fracturing. They have also been proposed for other uses involving the controlled transport of particulate materials. Examples of such proposed uses can be found in U.S. Pat. No. 6,435,277; U.S. Pat. No. 6,482,866; WO 2005/014975 and WO 2007/119211 which discloses additives for viscoelastic surfactant fluids and oilfield treatment methods comprising the injection of a viscoelastic surfactant, a poly-ethyleneglycol-propyleneglycol block copolymer and polynapthalene sulfonate. Further aspects of the use of visco-elastic surfactants can be found in ‘Expanding Applications for Viscoleastic Surfactants’, Oilfield Review Winter 2004/2005.
WO2004/057155 describes a method for fracturing a subterranean formation comprising injection of an aqueous fluid comprising a viscoelastic surfactant and a polymer additive.
US2003/221832 discloses a spacer fluid for use between a hydraulic cement slurry and a drilling fluid. However, no use of a viscoelastic fluid is mentioned.